The hidden trade is worth about $400 million a year — about two and a half times what Israel sold to its official Arab trading partners, Egypt and Jordan, in 2004 — said Gil Feiler, the director of Info-Prod Research, a Tel Aviv consultancy specializing in Arab markets, and an economics professor at Bar Ilan University.

Others say such estimates are significantly inflated.

"All the figures are very sexy for the press, but the reality is much less than what is written," said Dan Catarivas, foreign trade director at the Israeli Manufacturers' Association.

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The true amount of Arab imports from Israel is impossible to establish because neither side makes it public, with Israeli-made goods moving to Arab customers through third countries — Cyprus or the Netherlands, for example, which list the shipments as local exports.

An Arab lawyer who specializes in trade, Omar Obeidat of Al-Tamimi & Co. in Dubai, said the Arab League boycott of Israel is well enforced, despite the hidden trade through third countries.

"The only person who can confirm is the Israeli party to this covert operation," Obeidat said, when asked to estimate the worth of goods flowing to Arab nations.

Feiler, who has written a business guide to Israel in Arabic, refused to give more than rough outlines of the trade, which he supports.

Israeli exports to Arab countries, he said, are mostly from three categories: agricultural equipment, animal vaccines and "technological knowledge and components."